libexpr: Convert Symbol comparisons to switch statements

Now that Symbols are statically allocated at compile time with known IDs,
we can use switch statements instead of if-else chains for Symbol comparisons.
This provides better performance through compiler optimizations like jump tables.

Changes:
- Add public getId() method to Symbol class to access the internal ID
- Convert if-else chains comparing Symbol values to switch statements
  in primops.cc's derivationStrictInternal function
- Simplify control flow by removing the 'handled' flag and moving the
  default attribute handling into the switch's default case

The static and runtime Symbol IDs are guaranteed to match by the
copyIntoSymbolTable implementation which asserts this invariant.

Co-authored-by: John Ericson <git@JohnEricson.me>

src/libexpr/include/nix/expr/symbol-table.hh

@@ -61,6 +61,15 @@ public:
         return id > 0;
     }
 
+    /**
+     * The ID is a private implementation detail that should generally not be observed. However, we expose here just for
+     * sake of `switch...case`, which needs to dispatch on numbers. */
+    [[gnu::always_inline]]
+    constexpr uint32_t getId() const noexcept
+    {
+        return id;
+    }
+
     constexpr auto operator<=>(const Symbol & other) const noexcept = default;
 
     friend class std::hash<Symbol>;

src/libexpr/primops.cc

@@ -1454,19 +1454,22 @@ static void derivationStrictInternal(EvalState & state, std::string_view drvName
                     continue;
             }
 
-            if (i->name == state.s.contentAddressed && state.forceBool(*i->value, pos, context_below)) {
+            switch (i->name.getId()) {
+            case EvalState::s.contentAddressed.getId():
+                if (state.forceBool(*i->value, pos, context_below)) {
                     contentAddressed = true;
                     experimentalFeatureSettings.require(Xp::CaDerivations);
                 }
-
+                break;
-            else if (i->name == state.s.impure && state.forceBool(*i->value, pos, context_below)) {
+            case EvalState::s.impure.getId():
+                if (state.forceBool(*i->value, pos, context_below)) {
                     isImpure = true;
                     experimentalFeatureSettings.require(Xp::ImpureDerivations);
                 }
-
+                break;
             /* The `args' attribute is special: it supplies the
                command-line arguments to the builder. */
-            else if (i->name == state.s.args) {
+            case EvalState::s.args.getId():
                 state.forceList(*i->value, pos, context_below);
                 for (auto elem : i->value->listView()) {
                     auto s = state
@@ -1475,11 +1478,10 @@ static void derivationStrictInternal(EvalState & state, std::string_view drvName
                                  .toOwned();
                     drv.args.push_back(s);
                 }
-            }
+                break;
-
             /* All other attributes are passed to the builder through
                the environment. */
-            else {
+            default:
 
                 if (jsonObject) {
 
@@ -1488,49 +1490,69 @@ static void derivationStrictInternal(EvalState & state, std::string_view drvName
 
                     jsonObject->structuredAttrs.emplace(key, printValueAsJSON(state, true, *i->value, pos, context));
 
-                    if (i->name == state.s.builder)
+                    switch (i->name.getId()) {
+                    case EvalState::s.builder.getId():
                         drv.builder = state.forceString(*i->value, context, pos, context_below);
-                    else if (i->name == state.s.system)
+                        break;
+                    case EvalState::s.system.getId():
                         drv.platform = state.forceStringNoCtx(*i->value, pos, context_below);
-                    else if (i->name == state.s.outputHash)
+                        break;
+                    case EvalState::s.outputHash.getId():
                         outputHash = state.forceStringNoCtx(*i->value, pos, context_below);
-                    else if (i->name == state.s.outputHashAlgo)
+                        break;
+                    case EvalState::s.outputHashAlgo.getId():
                         outputHashAlgo = parseHashAlgoOpt(state.forceStringNoCtx(*i->value, pos, context_below));
-                    else if (i->name == state.s.outputHashMode)
+                        break;
+                    case EvalState::s.outputHashMode.getId():
                         handleHashMode(state.forceStringNoCtx(*i->value, pos, context_below));
-                    else if (i->name == state.s.outputs) {
+                        break;
-                        /* Require ‘outputs’ to be a list of strings. */
+                    case EvalState::s.outputs.getId(): {
+                        /* Require 'outputs' to be a list of strings. */
                         state.forceList(*i->value, pos, context_below);
                         Strings ss;
                         for (auto elem : i->value->listView())
                             ss.emplace_back(state.forceStringNoCtx(*elem, pos, context_below));
                         handleOutputs(ss);
+                        break;
+                    }
+                    default:
+                        break;
                     }
 
-                    if (i->name == state.s.allowedReferences)
+                    switch (i->name.getId()) {
+                    case EvalState::s.allowedReferences.getId():
                         warn(
                             "In a derivation named '%s', 'structuredAttrs' disables the effect of the derivation attribute 'allowedReferences'; use 'outputChecks.<output>.allowedReferences' instead",
                             drvName);
-                    if (i->name == state.s.allowedRequisites)
+                        break;
+                    case EvalState::s.allowedRequisites.getId():
                         warn(
                             "In a derivation named '%s', 'structuredAttrs' disables the effect of the derivation attribute 'allowedRequisites'; use 'outputChecks.<output>.allowedRequisites' instead",
                             drvName);
-                    if (i->name == state.s.disallowedReferences)
+                        break;
+                    case EvalState::s.disallowedReferences.getId():
                         warn(
                             "In a derivation named '%s', 'structuredAttrs' disables the effect of the derivation attribute 'disallowedReferences'; use 'outputChecks.<output>.disallowedReferences' instead",
                             drvName);
-                    if (i->name == state.s.disallowedRequisites)
+                        break;
+                    case EvalState::s.disallowedRequisites.getId():
                         warn(
                             "In a derivation named '%s', 'structuredAttrs' disables the effect of the derivation attribute 'disallowedRequisites'; use 'outputChecks.<output>.disallowedRequisites' instead",
                             drvName);
-                    if (i->name == state.s.maxSize)
+                        break;
+                    case EvalState::s.maxSize.getId():
                         warn(
                             "In a derivation named '%s', 'structuredAttrs' disables the effect of the derivation attribute 'maxSize'; use 'outputChecks.<output>.maxSize' instead",
                             drvName);
-                    if (i->name == state.s.maxClosureSize)
+                        break;
+                    case EvalState::s.maxClosureSize.getId():
                         warn(
                             "In a derivation named '%s', 'structuredAttrs' disables the effect of the derivation attribute 'maxClosureSize'; use 'outputChecks.<output>.maxClosureSize' instead",
                             drvName);
+                        break;
+                    default:
+                        break;
+                    }
 
                 } else {
                     auto s = state.coerceToString(pos, *i->value, context, context_below, true).toOwned();
@@ -1541,21 +1563,32 @@ static void derivationStrictInternal(EvalState & state, std::string_view drvName
                         drv.structuredAttrs = StructuredAttrs::parse(s);
                     } else {
                         drv.env.emplace(key, s);
-                        if (i->name == state.s.builder)
+                        switch (i->name.getId()) {
+                        case EvalState::s.builder.getId():
                             drv.builder = std::move(s);
-                        else if (i->name == state.s.system)
+                            break;
+                        case EvalState::s.system.getId():
                             drv.platform = std::move(s);
-                        else if (i->name == state.s.outputHash)
+                            break;
+                        case EvalState::s.outputHash.getId():
                             outputHash = std::move(s);
-                        else if (i->name == state.s.outputHashAlgo)
+                            break;
+                        case EvalState::s.outputHashAlgo.getId():
                             outputHashAlgo = parseHashAlgoOpt(s);
-                        else if (i->name == state.s.outputHashMode)
+                            break;
+                        case EvalState::s.outputHashMode.getId():
                             handleHashMode(s);
-                        else if (i->name == state.s.outputs)
+                            break;
+                        case EvalState::s.outputs.getId():
                             handleOutputs(tokenizeString<Strings>(s));
+                            break;
+                        default:
+                            break;
                         }
                     }
                 }
+                break;
+            }
 
         } catch (Error & e) {
             e.addTrace(